Phase 2: Implement Execution Environment Abstraction (v0.3.0)

This commit implements Phase 2 of the CHORUS Task Execution Engine development plan,
providing a comprehensive execution environment abstraction layer with Docker
container sandboxing support.

## New Features

### Core Sandbox Interface
- Comprehensive ExecutionSandbox interface with isolated task execution
- Support for command execution, file I/O, environment management
- Resource usage monitoring and sandbox lifecycle management
- Standardized error handling with SandboxError types and categories

### Docker Container Sandbox Implementation
- Full Docker API integration with secure container creation
- Transparent repository mounting with configurable read/write access
- Advanced security policies with capability dropping and privilege controls
- Comprehensive resource limits (CPU, memory, disk, processes, file handles)
- Support for tmpfs mounts, masked paths, and read-only bind mounts
- Container lifecycle management with proper cleanup and health monitoring

### Security & Resource Management
- Configurable security policies with SELinux, AppArmor, and Seccomp support
- Fine-grained capability management with secure defaults
- Network isolation options with configurable DNS and proxy settings
- Resource monitoring with real-time CPU, memory, and network usage tracking
- Comprehensive ulimits configuration for process and file handle limits

### Repository Integration
- Seamless repository mounting from local paths to container workspaces
- Git configuration support with user credentials and global settings
- File inclusion/exclusion patterns for selective repository access
- Configurable permissions and ownership for mounted repositories

### Testing Infrastructure
- Comprehensive test suite with 60+ test cases covering all functionality
- Docker integration tests with Alpine Linux containers (skipped in short mode)
- Mock sandbox implementation for unit testing without Docker dependencies
- Security policy validation tests with read-only filesystem enforcement
- Resource usage monitoring and cleanup verification tests

## Technical Details

### Dependencies Added
- github.com/docker/docker v28.4.0+incompatible - Docker API client
- github.com/docker/go-connections v0.6.0 - Docker connection utilities
- github.com/docker/go-units v0.5.0 - Docker units and formatting
- Associated Docker API dependencies for complete container management

### Architecture
- Interface-driven design enabling multiple sandbox implementations
- Comprehensive configuration structures for all sandbox aspects
- Resource usage tracking with detailed metrics collection
- Error handling with retryable error classification
- Proper cleanup and resource management throughout sandbox lifecycle

### Compatibility
- Maintains backward compatibility with existing CHORUS architecture
- Designed for future integration with Phase 3 Core Task Execution Engine
- Extensible design supporting additional sandbox implementations (VM, process)

This Phase 2 implementation provides the foundation for secure, isolated task
execution that will be integrated with the AI model providers from Phase 1
in the upcoming Phase 3 development.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

vendor/go.opentelemetry.io/otel/trace/embedded/README.md

@@ -0,0 +1,3 @@
+# Trace Embedded
+
+[![PkgGoDev](https://pkg.go.dev/badge/go.opentelemetry.io/otel/trace/embedded)](https://pkg.go.dev/go.opentelemetry.io/otel/trace/embedded)

vendor/go.opentelemetry.io/otel/trace/embedded/embedded.go

@@ -0,0 +1,45 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+// Package embedded provides interfaces embedded within the [OpenTelemetry
+// trace API].
+//
+// Implementers of the [OpenTelemetry trace API] can embed the relevant type
+// from this package into their implementation directly. Doing so will result
+// in a compilation error for users when the [OpenTelemetry trace API] is
+// extended (which is something that can happen without a major version bump of
+// the API package).
+//
+// [OpenTelemetry trace API]: https://pkg.go.dev/go.opentelemetry.io/otel/trace
+package embedded // import "go.opentelemetry.io/otel/trace/embedded"
+
+// TracerProvider is embedded in
+// [go.opentelemetry.io/otel/trace.TracerProvider].
+//
+// Embed this interface in your implementation of the
+// [go.opentelemetry.io/otel/trace.TracerProvider] if you want users to
+// experience a compilation error, signaling they need to update to your latest
+// implementation, when the [go.opentelemetry.io/otel/trace.TracerProvider]
+// interface is extended (which is something that can happen without a major
+// version bump of the API package).
+type TracerProvider interface{ tracerProvider() }
+
+// Tracer is embedded in [go.opentelemetry.io/otel/trace.Tracer].
+//
+// Embed this interface in your implementation of the
+// [go.opentelemetry.io/otel/trace.Tracer] if you want users to experience a
+// compilation error, signaling they need to update to your latest
+// implementation, when the [go.opentelemetry.io/otel/trace.Tracer] interface
+// is extended (which is something that can happen without a major version bump
+// of the API package).
+type Tracer interface{ tracer() }
+
+// Span is embedded in [go.opentelemetry.io/otel/trace.Span].
+//
+// Embed this interface in your implementation of the
+// [go.opentelemetry.io/otel/trace.Span] if you want users to experience a
+// compilation error, signaling they need to update to your latest
+// implementation, when the [go.opentelemetry.io/otel/trace.Span] interface is
+// extended (which is something that can happen without a major version bump of
+// the API package).
+type Span interface{ span() }